1. Field of the Invention
The present invention relates to an ozone generating apparatus, and more particularly to an ozone generating apparatus which can generate high concentration ozone at high efficiency.
2. Description of the Related Art
FIG. 1A is a sectional view showing a conventional ozone generating apparatus of a so-called Otto-Plate type which is disclosed in, for example, Ozonizer Handbook, Ozonizer Ad Hoc Committee in the Institute of Electrical Engineers of Japan, (Corona Publishing Co., Ltd., 1960) p.249, and FIG. 1B is a front view of a left half of the ozone generating apparatus. In the drawings, reference numeral 1 means a power source, 2 is grounded metallic electrodes, and 3 is high-voltage electrodes opposed to the grounded electrodes 2 and connected to the power source 1, and high voltage is applied to the high-voltage electrodes 3. Further, reference numeral 4 means dielectrics (glass plates) mounted on surfaces of the grounded electrodes 2 and the high-voltage electrodes 3, 5 is a discharge space in which discharge is generated, and 6 is electrical insulating (dielectric) spacers to form the discharge spaces 5. Reference numerals 7 and 8 mean arrows respectively showing a gas supply port and gas exhaust ports, and 9 is an exhaust pipe to exhaust an ozonized gas. FIG. 2A is a sectional view showing another ozone generating apparatus of a so-called Lowther Plate type disclosed in, for example, S. D. Razumovskii et al., Ozone and its reactions with organic compounds, ELSEVIER, (1984), and FIG. 2B is a sectional view taken along line B--B of FIG. 2A. In the drawings, the same reference numerals are used for component parts having the same functions as those in FIGS. 1A and 1B, and descriptions thereof are omitted. Reference numeral 41 means ceramic layers applied onto the grounded electrodes 2 and 3, having the same function as that of the glass plates 4.
A description will now be given of the operation. In the conventional ozone generating apparatus, a gas exhausting hole is provided in the grounded electrode 2, the high-voltage electrode 3, and the dielectric plate 4 at their intermediate portions. In the above publication disclosing the Otto-Plate type of apparatus, no description is given of the spacer 6. However, as shown in FIG. 47, in order to ensure an interval (an air gap length) between the dielectrics 4, 4, the electrical insulating spacer is mounted around the discharge space 5 in reality in such a way that the spacer does not interfere with a gas inflow. An oxygen-containing raw gas is introduced in a direction of the arrow 7 from an entire circumference of a peripheral portion of the ozone generating apparatus. Then, oxygen is partially turned into ozone when the gas passes through the discharge space 5 in which the discharge is caused by high voltage supplied from the power source 1. As a result, the ozone-containing gas is taken out as an ozonized gas in a direction of the arrow 8 through the gas exhausting pipe 9 mounted at the intermediate portion.
In the discharge spaces 5, heat is generated due to the discharge. Consequently, if the gas passing through the discharge space 5 is not effectively cooled, a gas temperature in the discharge space 5 is increased, and an amount of ozone generation is reduced. Hence, the grounded electrodes 2 and the high-voltage electrodes 3 are cooled by electrical insulating liquid such as insulating oil, thereby reducing a rise of the gas temperature.
The ozone generating apparatus in FIGS. 2A and 2B has the same basic structure as that of the ozone generating apparatus shown in FIGS. 1A and 1B. In this case, the two ozone generating apparatus are different from an ozone generating apparatus shown in FIG. 47 in that the gas supply port and the gas exhaust port are separately mounted, and the gas flows in a direction shown in the drawings. Further, in the ozone generating apparatus shown in FIGS. 1A and 1B, the electrical insulating spacers 6 (made of, for example, silicone) are illustrated. The spacers 6 can ensure the interval (the air gap length) between the electrodes 2 and 3, and are used as sealing members to prevent gas leakage from the discharge spaces.
A description will now be given of a characteristic of the conventional ozone generating apparatus with reference to FIGS. 3 to 6. In the drawings, reference numeral Q.sub.N means a flow rate of the raw gas (converted according to Standard Temperature and Pressure [STP]), W is discharge power, C.sub.03 is an ozone concentration (converted according to STP) at the gas exhausting port of a discharge portion, T.sub.W is a temperature of cooling water, d is a discharge gap length, S is a discharge area between the electrodes 2 and 3, and .eta. is ozone yield. W/Q.sub.N means discharge power consumption per gas molecule, and serves as an important parameter of an ozone generation characteristic. W/S means discharge power (power density) per unit area of the discharge space between the electrodes 2 and 3, and serves as a parameter reflecting the gas temperature. The ozone yield .eta. means the amount of ozone generation per unit discharge power, and can be expressed as .eta.=C.sub.03 /(W/Q.sub.N). In view of performances (about a compact size and an efficiency) of the ozone generating apparatus, .eta. and W/S are preferably set to larger values, and C.sub.03 is also preferably set to a larger value.
FIG. 3 is a diagram showing a relationship between power consumption per molecule W/Q.sub.N and the ozone concentration C.sub.03 when the power density W/S and the discharge gap length d are kept constant, and the temperature of cooling water is varied. As set forth above, the power consumption per molecule W/Q.sub.N serves as a basic parameter related to ozone generation, and the ozone yield .eta. is more reduced as the power consumption W/Q.sub.N is more increased (in the drawing, the straight lines can be described when the ozone yield .eta. is constant, and the ozone yield .eta. becomes larger in the upper line). Further, the temperature T.sub.W of cooling water does not have a great effect when the power consumption W/Q.sub.N is small. However, when the power consumption W/Q.sub.N becomes larger, the ozone concentration C.sub.03 (and the ozone yield .eta.) becomes larger as the temperature T.sub.W of cooling water becomes lower. That is, for higher concentration ozone, it is important to set a low temperature of cooling water and keep a low gas temperature.
FIG. 4 shows a relationship between the power consumption W/Q.sub.N and the ozone concentration C.sub.03 when the temperature T.sub.W of cooling water and the discharge gap length d are kept constant, and the power density W/S is varied. It can be understood that an increase in the power consumption W/S results in the same effect as that obtained by an increase in the temperature T.sub.W of cooling water in FIG. 3. This is because the increase in the power consumption W/S and the increase in the temperature T.sub.W of cooling water can have the same effect on an increase in the gas temperature in the discharge space 5.
FIG. 5 shows the ozone concentration C.sub.03 with respect to the power consumption W/Q.sub.N when the temperature T.sub.W of cooling water and the power density W/S are kept constant, and the discharge gap length d is varied in the range from 0.8 to 1.6 mm. An increase in the discharge gap length d can provide an effect which is very similar to the effect obtained by the increase in the temperature T.sub.W of cooling water.
Here, when an average gas temperature eav in the discharge space is defined as the following expression (1), another average gas temperature in the discharge space of the ozone generating apparatus in which the only single side of the electrodes is cooled can be represented by the expression (2). Further, the expression (3) can be held when both sides of the electrodes are cooled. ##EQU1## where x denotes a distance in an air gap direction, d is the discharge gap length, .theta.(x) is the gas temperature in case of the distance x, ka is coefficient of heat transfer of the gas, and T.sub.W is the temperature of cooling water.
From the expressions (1) to (3), though different coefficients are required according to methods of cooling the electrode, it can be seen that the average gas temperature .theta..sub.av is proportional to the discharge power density W/S and the air gap length d. That is, when the air gap length d is set to a small value, the average gas temperature .theta..sub.av can be limited to a lower value even if constant power is supplied, resulting in the high concentration ozone as in the case of d=0.8 mm in FIG. 5. However, when the set air gap length d is extremely short, and a plurality of ozone generating units are provided to have a multi-stage structure, a larger variation is generated in the air gap length d of the discharge space of the ozone generating units. Therefore, another variation is caused in the flow rates Q.sub.N of gases passing through the discharge spaces, thereby causing still another variation in the discharge power W supplied into the discharge spaces. As a result, the equivalent power consumption W/Q.sub.N is increased so that an ozone generating efficiency is reduced as shown in FIGS. 3 to 5. Further, as shown in FIG. 6, it is known that an excessively small air gap length d lowers an ozone exciting efficiency itself. FIG. 6 is a diagram illustrated in Czech, J. Phys., B38, (1988), FIG. 7, p.648, in which the transverse axis defines the air gap length, and the ordinate axis defines the ozone generating efficiency. Further, the symbols O and +respectively represent the results of two cases, that is, one case where air is used as the raw gas and the other case where oxygen is used as the raw gas. The article teaches that the optimal air gap length for the ozone generation is in an approximate range of 0.8 to 1.1 mm (see the first line on page 645). In particular, it is emphasized that a narrow air gap of 0.6 mm or less reduces the ozone exciting efficiency. Hence, in the conventional ozone generating apparatus, the air gap length d is set in the range of 0.8 to 1.5 mm, and a thermal problem is avoided by operation with the power density W/S in a low range. That is, the apparatus is designed to have a large form and have a large discharge area, thereby improving the ozone generating efficiency.
The conventional ozone generating apparatus is provided as set forth above. As a result, there are problems in that, for example, a low gas temperature in the discharge space should be held, and for this purpose, the power density W/S should be reduced by providing the large ozone generating apparatus and the large discharge area S.